Electric discharge tube



April 1934- K. M. VAN GESSEL ET AL 1,954,139

ELECTRIC DISCHARGE TUBE Filed Nov. 15, 1929 :Znentorsr /far-/ llarznqs l Gcsse? Patented Apr. 10, 1934 ELECTRIC DISCHARGE TUBE Karel Marinus Van Gessel and Wilhclmus Daniel van Wijk, Eindhoven, Netherlands, assignors, by mesne assignments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware Application November 15, 1929, Serial No. 407,561 In the Netherlands November 30, 1928 3 Claims.

During this heating operation which is often eflected prior to the filaments being mounted in the discharge tubes, tungsten filaments become brittle so that their mechanical strength is materially decreased and their mounting in the discharge tubes is rendered very difiicult.

According to the invention which is concerned with an electric discharge tube, the said disadvantages are obviated when the filament which is enclosed in the said discharge tube is not entirely of tungsten but consists of a body having a sheath of tungsten. Preferably, the core of the filament contains molybdenum and the filament may be coated at least partially with a substance of high electronic emission. The core of the filament may 5 contain an oxide of thorium or of another material of high electronic emission.

Though the tungsten sheath of the filament used according to the invention becomes brittle by heat, a sufiiciently strong filament body may be obtained by a suitable choice of the material of which the core of the filament is constituted and of the thickness of the tungsten sheath. Preferably, the thickness of the tungsten sheath is less than 15% of the thickness of the filament, say

The invention will be more clearly understood by reference to the accompanying drawing in which an embodiment of the invention is illustrated.

As the starting material for the filament of the discharge tube to be produced, a material may be taken which becomes brittle at a higher temperature than tungsten, for example a molybdenum wire which may have a diameter of 1500s. This wire is coated with a film of tungsten, for example,

' of such thickness that the sheath wire thus produced has a diameter of 1580 This operation of coating with tungsten may be effected by heating the molybdenum wire to say 1700 C. in an 9 atmosphere which contains a volatile tungsten compound, for example, an atmosphere which contains tungsten hexachloride. The sheath wire thus produced may be drawn down into a wire of smaller diameter. It is preferable that the drawn wire before being subjected to further handlings should be examined by immersion in a liquid which attacks the material of which the core is constituted but which does not attack tungsten. The points at which the core material is not covered with tungsten can thus be rendered visible so that it may be ascertained to which extent the core material is coated with tungsten.

If the core of the sheath wire is composed of molybdenum, the wire may be immersed, for example, in a mixture of concentrated sulfuric acid and strong nitric acid.

It has been found that a filament produced in the manner described can be heated to a higher temperature than a filament entirely composed of tungsten without its mechanical strength being materially lowered. Thus, the body can be deprived of occluded gases prior to its being mounted in a discharge tube without becoming brittle and can be subjected without difficulty to the necessary deformations.

The filament can be coated in any known I manner with a substance of high electronic emission, for example, with an earth alkaline metal or an earth alkaline metal oxide and is thus adapted to be used as a Wehnelt cathode. The earth alkaline metal may be precipitated, for example, on the filament from the vapour which may be developed in the tube by decomposition of an earth alkaline metal compound which consists, for example, of barium azide and which is mounted in the tube, for example, on an anode. If desired, a further material, for example copper, may be mounted between the tungsten sheath and the electron emitting substance. An advantage of the cathode described compared with a cathode constituted of a molybdenum wire coated directly with an earth alkaline metal consists in that no compounds are formed between the earth alkaline metal and the molybdenum.

If the material by which the core is constituted has added to it thorium oxide or another substance which may produce on the cathode a film of high electronic emission and if this core is coated with a tungsten sheath, the filament thus produced and used as an incandescent cathode is found to disintegrate to a much smaller extent than a filament which is not provided with a tungsten sheath such, for example, as a thoriated molybdenum cathode, and the filament is less brittle than a thoriated tungsten cathode although it is possessed of a like emission and disintegration as the latter so that it may be substituted satisfactorily for the cathode of the well known Tungar rectifier.

The discharge tubes according to the invention,

which may be exhausted, may be used for vari ous purposes, for example, for receiving and amplifying electric oscillations, as a transmitter valve, for rectifying alternating currents. The filaments described may be used with advantage in discharge tubes filled with gas, for example, a rare gas.

In the accompanying drawing an example of an electric discharge tube according to the invention is shown. The tube illustrated in Figure 1 comprises a bulb 1 of glass or similar material, to which a stem 2 has been fused. This stem carries the electrodes of the tube consisting preferably of a cylindrical anode 3 of nickel or other metal and an electron emitting cathode 4. The anode is mounted upon the stem with the aid of the supporting wire 5, Whereas the cathode is supported by the wires 6 and'7. The cathode contains, as can be seen from Figure 2, a composite wire comprising a core 8 of molybdenum and a sheath 9 of tungsten. Upon this composite wire an electron emitting coating 10 consisting for instance of barium oxide is formed.

What we claim is:

1. An electric discharge tube, free from occluded gases, comprising an envelope, an anode,

and a cathode, said cathode comprising compound wire comprised of a core of molybdenum and a coating of tungsten substantially entirely covering said core, the thickness of said tungsten being less than approximately 15% of the thickness of said wire, and a coating of oxide on said Wire.

2. An electric discharge tube, free from occluded gases, comprising an envelope, an anode, and a cathode, said cathode comprising compound wire comprised of a core of molybdenum and a coating of tungsten substantially entirely covering said core, the thickness of said tungsten being approximately 15% of the thickness of said Wire, and a coating of oxide on said wire.

3. A Wehnelt' cathode for an electric discharge tube, comprising a molybdenum core, a coating of tungsten substantially covering said core, the thickness of said tungsten being less than approximately 15% of the thickness of said wire, and a coating of oxide on said wire, said cathode remaining ductile at the temperatures required for degassing as Well as at the operating temperatures of the cathode.

KAREL MARINUS VAN GESSEL.

WILHELMUS DANIEL VAN WIJK. 

